The present invention relates to a device for growing single crystals, in which crystal material is melted in a crucible, a crystal nucleus or seed crystal is immersed in the molten crystal material and slowly pulled out of the melt, so that further material continuously crystallizes on the crystal nucleus and forms the single crystal. The present invention further relates to a device for carrying out the crystal-growing process.
The mineral corundum Al2O3 has a trigonal-skalenoid symmetry and generally forms crystals of prismatic, pyramidal, tabular, or rhomboid habitus. An example of a corundum crystal is the sapphire. Sapphires are gems occurring in nature, made from aluminum oxide Al2O3 with a corundum structure. Sapphires generally have an iron content which lies between 0.005 and 0.8% Fe and contain small quantities of titanium in oxidation stage +4. In industry, synthetically produced colorless corundum single crystals are also referred to as sapphires.
Recently, 0° sapphire wafers have been used as carrier substrates for the growth of GaN, InGaN and similar semiconductor layers for the production of LEDs, in particular of blue-white LEDs. Here 0° means that the crystallographic c-axis is oriented perpendicular to the wafer surface. 0° sapphire wafers are particularly suitable for epitaxy, as they possess a suitable lattice constant and suitable surface properties, so that epitaxial growth of the desired semiconductor layers can take place easily.
In particular in the production of LEDs it is an advantage if the crystal material, i.e. in particular the sapphire wafer used as a base, has a few crystal structure defects as possible, as the yield of good LEDs per wafer can thereby be increased, which in turn contributes to a reduction in costs.
However, sapphire possesses anisotropic physical properties. For example, the linear thermal expansion coefficient behaves differently in the two main crystallographic directions. If a sapphire crystal is heated for example from 20° C. to 980° C., the lattice constant changes in the a-direction by approximately 0.8%, whilst the lattice constant in the c-direction changes by approximately 0.9%. With the known crystal growing processes, this can lead to problems which reduce the quality of the synthetically produced sapphires.
For the production of sapphire crystals—mainly for the watch glass industry—hitherto the Verneuil process has chiefly been used. In this process, aluminum oxide powder is trickled through an oxyhydrogen flame and melted to form drops, which settle on a sapphire seed arranged beneath, and grow there. With this process, sapphire crystals with a diameter of up to approximately 40 mm are grown, which however generally have small-angle grain boundaries and other crystal structure defects.
At present, sapphire crystals for GaN epitaxy are essentially grown according to the following four different processes:
Czochralski process: In this process, a seed crystal is immersed in melt present in a crucible and then pulled out again, in self-supporting manner, whilst precisely controlling the rate of pulling. As the melt has a temperature above, and the seed crystal a temperature below the melting temperature of the crystal, at a suitably selected rate of pulling, crystal material crystallizes on the seed crystal. In this process however a relatively high thermal gradient is formed over the crystal already pulled out of the melt. This can give rise to lattice structure defects in the crystal—caused by the anisotropic linear thermal expansion coefficients.
In the known devices for carrying out the Czochralski crystal pulling process, the crystal material is generally melted in a crucible serving as a susceptor, by means of an induction coil serving as an inductor, with the crucible susceptor being heated by the inductor, so that, in turn, the material present in the crucible is heated. As no other heating elements are provided, a very high temperature gradient inevitably forms along the crystal grown.
A large number of tests have shown that the poor quality, in particular the large number of crystal structure defects in the crystals produced with the standard Czochralski process, is essentially due to the relatively high thermal gradients, in particular in the pulling direction, i.e. in the axial direction of the crystal oriented in the c-direction.
In order to minimize this effect in the crystal during this growing process, the seed crystal is oriented so that the crystallographic c-axis is at an angle of 60° or 90° to the pulling direction. Although, with this process, relatively high-grade crystals are produced, the crystal yield for wafers oriented along the c-axis is however very small, as ingots, i.e. blocks of crystal have to be drilled sideways out of the crystals produced in this way, for wafer production. This process is therefore very time-consuming and expensive.
Nacken-Kyropoulos process: In this process, 90° sapphire crystals, i.e. crystals, in which the pulling direction runs perpendicular to the c-axis of the crystal, are grown with relatively large diameters. In order to obtain 0° wafer ingots, corresponding cylinders then have to be drilled out of the crystals produced, perpendicular to the pulling direction in the direction of the crystallographic c-axis, and then sawn up in order to obtain wafers. This growing process is extremely labor-intensive and has only a small material yield.
Bagdasarov process: The sapphire single crystals grown by means of this process have a bar-shaped expansion, with the 0° direction, i.e. the crystallographic c-axis, usually lying in the direction of the smallest dimension, which is itself generally smaller than 30 mm, with an optimum process development. Although corresponding 0° wafer ingots can also be drilled out in this case, these are however so short, that efficient further processing to produce wafers is scarcely economic.
Heat-exchanger method: In the heat-exchanger method, which is also referred to as HEM, the raw material, melted in a crucible, is solidified from the crucible floor upwards by suitable cooling. By placing seed crystals on the crucible floor, the crystal orientation can be predetermined. With this growing method, although quite large and qualitatively high-grade crystals can be produced, the time required is very considerable.